(1) Field of the Invention
The present invention relates to a process for preparing polyesters, especially polyethylene terephthalate, by continuously carrying out polymerization by using a thin film polymerization apparatus, and also to an apparatus for use in the practice of this process.
(2) Description of the Related Art
Synthetic polyester polymers have excellent physical and chemical properties, and are widely used in various fields. Especially, polyethylene terephthalate has an excellent chemical resistance, heat resistance, insulation resistance, gas barrier property, tenacity, and elasticity modulus and is used in large quantities for textile fibers, industrial fibers, films, containers, and ordinary shaped articles. In order to enjoy greater advantages and reduce production costs, the changeover of batchwise polymerization to continuous polymerization has been tried and advanced. Since the uses of polyesters have been recently diversified, it is now necessary to produce a variety of polymers suitable for respective uses, individually and in small quantities. In order to prepare polymers suitable for diversified uses while utilizing the merits of the continuous polymerization system, the use of a thin film polymerization apparatus having a small capacity is considered advantageous. For example, there have been proposed a process in which a prepolymer from a polymerization vessel at the stage preceding the final stage is fed to at least two final polymerization vessels connected in parallel to each other, a general-purpose polymer is produced in a large quantity by a conventional lateral polymerization reactor, and a special polymer is prepared by an easily exchangeable, small-capacity thin film polymerization reactor (tank), as disclosed, for example, in Japanese Unexamined Patent Publication No. 58-96627, and a process in which monomers and/or oligomers are produced in large quantities, are fed into several lines in parallel at the polymerization steps, one small-capacity thin film polymerization apparatus (tank) or a plurality of such apparatuses connected in series is used in each line, and special polymers are prepared in the respective lines.
The following reaction methods are adopted in this thin film polymerization apparatus.
(1) Stirring vanes are arranged and stirring is carried out at a high speed.
(2) A natural flow-down film is formed by using a wet wall.
(3) A great number of fine filamentary materials are formed to increase the area of the surface for evaporation of volatile by-products.
In the method (1), by forcibly stirring the reaction mixture, the surface of the reaction mixture is renewed and separation of volatile by-products is easily accomplished and a high reaction speed can be obtained, and this method is especially preferred. However, this method is defective in the following points.
(A) Where the apparatus has a central shaft and stirring vanes are attached to this central shaft, the shaft- and stirring vane-attaching portions are not wetted with the polymer and so-called dead spaces are formed, and materials scattered to these dead spaces are converted to gel-like foreign substances with the lapse of time.
(B) When stirring vanes are arranged in a cage-like cylinder having no central shaft, adhesion of a polymer to stirring vane-attaching portions is reduced, but since there is no central shaft, any increase in the rotation speed is restricted by the structural strength limits. Accordingly, renewal of the surface by shearing a thin film polymer is unsatisfactory, and it is difficult to greatly increase the reaction speed.
(C) As a means for eliminating dead spaces, there has been proposed a mechanism making an epicyclic movement, as disclosed in Japanese Examined Patent Publication No. 48-13240. However, this apparatus is defective in the following points.
(i) Since a vane is in contact with the wall, high-speed rotation is impossible and the reaction speed cannot be sufficiently increased. PA1 (ii) Since the direction of revolution is the reverse of the direction of rotation, and the peripheral speeds counteract each other, and the reaction speed cannot be sufficiently increased. PA1 (iii) Since many driving gears must be disposed and ball joints arranged in the reaction tank to maintain the wall-vane contact, the mechanism becomes complicated.